The present invention relates to a method for the preparation of a double-sided flexible circuit board with electrical connection between the printed circuit patterns on both surfaces of a substrate at a through-hole or an opening penetrating the substrate. More particularly, the invention relates to a convenient and very reliable method for the preparation of a double-sided flexible circuit board having two circuit patterns printed with an electroconductive paste on both surfaces of a flexible substrate and electrically connected by way of a through-hole formed in the substrate. Such a double-sided circuit board is referred to as a circuit board with through-hole hereinbelow.
Double-sided circuit boards with through-hole are manufactured conventionally by punching a rigid circuit board provided on one surface with a completed circuit pattern to form a through-hole and then providing an electroconductive coating layer on and around the through-hole by applying a conductive paint or by metal-plating followed by forming another circuit pattern on the other surface of the substrate board. This method has several problems including breaking of wiring and troubles in the coating or plating process which is usually complicated and sometimes causes environmental pollution.
A method is proposed in Japanese Patent Publication 57-19876 according to which a flexible resin film as a substrate provided with a circuit pattern on one surface by printing is punched to make an opening having a diameter of 0.1 to 2 mm and the peripheral or side surface of the opening is coated with an electroconductive paste followed by printing of a circuit pattern on the other surface of the substrate film by the method of screen printing to establish electric conduction between two circuit patterns on the surfaces by means of the conductive paste on the side surface of the opening. An improvement of this method is proposed in Japanese Patent Publication 58-20158 according to which the side surface of the opening coated with the conductive paste is overcoated with another conductive paste of a relatively low viscosity with an object to improve the reliability of electric conduction via the side surface of the opening.
Alternatively, Japanese Patent Kokai 60-208888 proposes a method for the preparation of a flexible circuit board with through-hole, according to which a base film as the substrate provided on one surface with a circuit pattern by printing is punched from the other surface to make an opening which is filled with a conductive paste injected at the surface without the circuit pattern. Further, Japanese Patent Kokai 60-208894 proposes a method in which a base film as the substrate provided on one surface with a first circuit pattern by printing is provided with an opening which is filled with a conductive paint and then a second circuit pattern is formed on the other surface of the substrate by using a conductive paint.
The above described prior art methods are each a method in which an opening formed in the substrate is filled or coated with an electroconductive paste or paint so that the reliability of the method largely depends on the viscosity or rheological properties of the paste or paint sometimes requiring repetition of the filling or coating works with the paste or paint. When an excessively large volume of the conductive paste or paint is injected into the opening, moreover, the extraneous portion of the paste or paint necessarily overflows out of the opening to badly contaminate the surface of the substrate film resulting in failure of the electric insulation.